The invention relates to a process for synthetizing zeolites having a mordenite structure and a high catalytic activity, in which the molar ratio of SiO.sub.2 to Al.sub.2 O.sub.3 is equal to at least 16; this kind of zeolites is referred to hereinafter as ME zeolites, ME mordenites or also as ME mordenitic zeolites.
It is known to introduce into the tridimensional structure of the zeolites, in the course of the synthesis step, an organic cation, for example a quaternary ammonium cation, such as tetrapropyl-ammonium, tetrabutyl-ammonium or diethyl-piperidinium, and it is known that these cations lead to the obtainment of particular original structures; see, for example, European patent publication 21,445, in the name of the Applicant. The inorganic cations are easily exchangeable, either totally or partially, with other cations, while the organic cations originally present are not susceptible of any further cationic exchange due to the reduce dimensions of their pores; zeolite, once it has been dehydrated, exhibits absorption characteristics.
Out of the zeolites having a pentasilicate-ring structure, a known class is the one of mordenites; they exist in nature (mordenite small ports), but are also prepared by synthesis (mordenite large ports) and the preparation thereof is known for example from R. M. Barrer "Molecular Sieves" page 39 Soc. Chem. Ind. (1968). According to U.S. Pat. No. 3,436,174, a mixture containing an Al source (e.g. Na aluminate), a Si source (e.g. Na silicate) and an alkali metal source (e.g. soda or just the silicate or aluminate) is put into H.sub.2 O, in defined proportions and at a suitably adjusted pH. The mixture is hydrothermally treated (with or without pressure) for a time and at a temperature such as to promote crystallization, the resulting mordenite having a general formula, for example, of the type: EQU M.sub.p/n [(AlO.sub.2).sub.p (SiO.sub.2).sub.48-p ].XH.sub.2 O
wherein M is a cation and n the valence thereof and where p is usually such, that the molar ratio of SiO.sub.2 to Al.sub.2 O.sub.3 ranges from 8 to 10.
By increasing the SiO.sub.2 :Al.sub.2 O.sub.3 ratio in the starting gel, besides obtaining mordenite with a final ratio of SiO.sub.2 to Al.sub.2 O.sub.3 equal to 10, other silicates of the type of analcite and .alpha.-quartz crystallize (R. M. Barrer et al., J. Chem. Soc. 1952, page 1561; P. K. Baipai et al., Ind. Eng. Chem. P.R.D. Vol. 17, pages 223, 1978).
The conditions for synthetizing a mordenite having a high SiO.sub.2 /Al.sub.2 O.sub.3 ratio as a single crystalline phase are very critical and depend (besides on the organic base selected) on the temperature, the pH and the type of alkalinity existing, these structures being the result of complex nucleation, crystallization and recrystallization reactions in systems far from the equilibrium; see A. Erdem and L. B. Sand J. Catalysis, Vol. 60, page 241 (1978); in general, crystalline multiphase systems are obtained.
The substitution of a part of the inorganic base by a strong organic base renders the forming of crystalline SiO.sub.2 less easy, at least in the range of the claimed temperatures and compositions, thus allowing the nucleation of mordenite crystals also at SiO.sub.2 /Al.sub.2 O.sub.3 ratios higher than 10. In these conditions and with specific organic bases, closely related structures may form; that is the case of ZSM-5, ZSM-11 and ZSM-8 zeolites, characterized by silicate subunits similar to the ones of mordenite.
By introducing particular organic components into the crystallization mixture, mordenites having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of from 15 to 30 have been produced by direct synthesis, and tetraethyl-ammonium salts (U.S. Pat. No. 4,052,472) or derivatives of neopentylamine (EP 14023) have been employed; the mordenites prepared according to this method reveal, under the electron microscope, a characteristic morphological aspect (EP 14544).